In recent years, thermal transfer systems have been developed to obtain prints from pictures which have been generated electronically from a color video camera. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converted into electrical signals. These signals are then operated on to produce cyan, magenta and yellow electrical signals. These signals are then transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face-to-face with a dye-receiving element. The two are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to one of the cyan, magenta or yellow signals, and the process is then repeated for the other two colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen. Further details of this process and an apparatus for carrying it out are contained in U.S. Pat. No. 4,621,271, the disclosure of which is hereby incorporated by reference.
Commonly-used dyes are nonionic in character because of the easy thermal transfer achievable with this type of compound. The dye-receiver layer usually comprises an organic polymer with polar groups to act as a mordant for the dyes transferred to it. A disadvantage of such a system is that since the dyes are designed to be mobile within the receiver polymer matrix, the prints generated can suffer from dye migration over time.
There is a need to provide thermal dye transfer imaging systems that yield images which are resistant to degradation by contact with other surfaces, chemicals, fingerprints, etc. Such image degradation is often the result of continued migration of the transferred dyes after the printing step.
One approach to reducing the continued migration of the transferred dyes is to utilize basic dyes which are capable of reacting with acidic substances in the receiving element to form migration-resistant cationic dye salts. However, it is difficult to effectively protonate a thermally transferred basic dye in a hydrophobic, polymeric receiving element with a conventional, electrically neutral acidic substance because the polymeric medium provides little stabilization of the charged dye salt that is formed.
One way to overcome this is to use a very strong protic acid such as a sulfonic acid. However, strong protic acids can have deleterious effects on many polymeric materials and often induce unwanted color shifts of the transferred dyes.